Due to its characteristics of being easily applicable to various products and electrical characteristics such as a high energy density, a secondary battery is not only commonly applied to a portable equipment, but universally applied to an electric vehicle (EV), a hybrid vehicle (HV), or an energy storage system that is propelled by an electric motor.
This secondary battery is gaining attention for its primary advantages of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy efficient source of energy.
FIG. 1 is an exploded perspective view schematically illustrating a configuration of a pouch-type secondary battery according to a related art. Referring to FIG. 1, the conventional pouch-type secondary battery 1 includes an electrode assembly 10 and a pouch case 20 as a basic structure.
Here, the electrode assembly 10 includes a cathode plate, an anode plate, and a separator interposed between the cathode plate and the anode plate to electrically isolate the cathode plate from the anode plate, and the electrode assembly 10 is equipped with a cathode tab extending from the cathode plate and an anode tab extending from the anode plate.
The cathode tab and the anode tab may be coupled to a cathode lead 11 and an anode lead 12 by a resistance welding method, a ultrasonic welding method, or a laser welding method, and the electrode lead is exposed through the pouch case to perform a function of electrically connecting the secondary battery to an external application device as an electrode of the secondary battery.
The electrode assembly 10 is introduced in the pouch case 20 together with an electrolyte solution.
The pouch case 20 may be classified into an upper case 21 and a lower case 22, and may be called a single cap or a double cap based on a location of a part in which the electrode assembly 10 is received.
The pouch case 20 has an aluminum thin film interposed therein to protect the electrolyte solution and the electrode assembly 10 disposed inside as well as to complement the electrochemical properties of a battery cell and improve heat resistance. In this instance, to ensure insulation of a battery cell from an external environment, the aluminum thin film may have, on an outer surface, an insulation layer coated with an insulation material, for example, polyethylene terephthalate (PET) resin or nylon resin.
During a sealing process, the pouch case 20 may be joined or adhered along its outer periphery by heat fusion. To do so, an adhesive layer of casted polypropylene (CPP) or polypropylene (PP) is formed on a lower surface of the upper case 21 and an upper surface of the lower case 22 to achieve adhesion therebetween. In addition to adhesion of the pouch case 20, the adhesive layer also serves as an insulation layer to prevent conduction between the aluminum layer and the electrolyte injected into the pouch case 20.
FIG. 2 is an enlarged cross-sectional view of sections A and B of FIG. 1. Referring to FIG. 2, the upper case 21 has a predetermined layered structure of an insulation layer 25, an aluminum layer 24, and an adhesive layer 23 in a sequential order, and the lower case 22 has a structure of an adhesive layer 23, an aluminum layer 24, and an insulation layer 25.
To seal the pouch case 20, heat and pressure may be provided to the lower adhesive layer 23 of the upper case 21 and the upper adhesive layer 23 of the lower case 22.
FIG. 3 is an enlarged cross-sectional view illustrating a state in which the aluminum layer is exposed to the electrolyte due to imprecise sealing of the sealing portion when sealing the pouch case according to the related art. Referring to FIG. 3, when erroneous sealing occurs at the sealing portion of the pouch case 20 as represented by C, the adhesive layer 23 may have a reduced thickness at the inner side of the sealing portion where the electrode assembly 10 is placed. If the adhesive layer 23 formed at the inner side of the sealing portion is not thick, a portion of the aluminum layer 24 may be exposed to the electrolyte as indicated by D.
This may occur when the adhesive layer 23 of the sealing portion is forced to move in a side direction, particularly, in an outer side direction of the secondary battery, by the pressure during the sealing process of the pouch case 20. Also, in this case, dielectric breakdown occurs to the secondary battery, causing serious safety problems such as combustion, explosion or fire due to an internal short circuit, as well as damage to the secondary battery.